1. Field of the Invention
The present invention relates to integrated semiconductor circuits. More particularly, the present invention relates to a CMOS low-power, wide-linear-range, well-input differential amplifier and transconductance amplifier incorporating a gate degeneration and/or a source degeneration feedback circuit.
2. The Prior Art
In the past few years, there have been many advances in improving the linearity of MOS transconductor circuits. See, e.g., H. Khorramabadi and P. R. Gray, "High Frequency CMOS continuous time filters", I.E.E.E.J. Solid State Circuits, 1984, SC-19(6), pp. 939-948; Y. Tsividis, Z. Czarnul, S. C. Fang, "MOS transconductors and integrators with high linearity", Electronics Letters, 22(5), 1986, pp. 245-246; B. Nauta and E. Seevinck, "Linear CMOS transconductance element for VHF filters", Electronics Letters, 25, 1989, pp. 448- 450; S. Szczepanski, J. Jakusz and A. Czarniak, "Differential Pair Transconductor Linearisation via Electronically Controlled Current-Mode Cells", Electronics Letters, 28(12), 1992, pp. 1093-1095.
These advances have primarily been in the area of above-threshold, high-power, high-frequency, continuous-time filters. Though it is possible to implement auditory filters (20 Hz-20 kHz) with these techniques, it is inefficient to do so. The transconductance and current levels in above-threshold operation are so high that large capacitances or transistors with very low W/L are required to create low-frequency poles, and area and power are wasted. In addition, it is hard to span three orders of magnitude of transconductance with a square law, unless transistors with ungainly aspect ratios are used. However, it is easier to obtain a wide linear range above threshold and the noise floor is lower because of the much higher charge and current density levels.
In above-threshold operation, identities like (x-a).sup.2 -(x-b).sup.2 =(b-a) (2x-a-b) are used to increase the wide linear range even further. In bipolar devices where the nonlinearity is exponential rather than second-order, it is much harder to completely get rid of the nonlinearity. The standard solution has been to use the feedback technique of emitter-degeneration which achieves wide linear range by reducing transconductance, and is described by P. R. Gray and R. G. Meyer, "Analysis and Design of Analog Integrated Circuits", 2nd. Ed., pp. 180-186, John Wiley and Sons, 1984. A clever scheme for widening the linear range of a bipolar transconductor, that cancels all nonlinearities up to the fifth order, without reducing the transconductance, has been proposed by G. Wilson, "Linearised Bipolar Transconductor", Electronics Letters, 28(4), 1992, pp. 390-391. A method for getting perfect linearity in a bipolar transconductor by using a translinear circuit and a resistor has been demonstrated by W. Chung, K. Kim and H. Cha, "A Linear Operational Transconductance Amplifier for Instrumentation Applications", I.E.E.E. Trans. on "Instrumentation and Measurement", 41(3), 1992, pp. 441-443. Both of the latter methods, however, require the use of resistors, and ultimately derive their linearity from the presence of a linear element in the circuit. Resistors, however, cannot normally be tuned electronically, and require special process steps.
Some authors have used an MOS device as the resistive element in an emitter-degeneration scheme to make a BiCMOS transconductor, e.g., the scheme proposed by J. Ramirez-Angulo and E. Sanchez-Sinecio, "Programmable BiCMOS Transconductor for Capacitor-Transconductor Filters", Electronics Letters, 28(13), 1992, pp. 1185-1187. Another BiCMOS technique, reported by W. Liu, "An Analog Cochlear Model: Signal Representation and VLSI Realization", Ph.D. Thesis, Johns Hopkins University, Baltimore, Md., 1992, uses an above-threshold differential pair to get wide linearity and scales down the output currents via a bipolar Golbert gain cell, to levels more appropriate for auditory frequencies. Above-threshold differential pairs, however, require techniques like cascode mirrors to improve the output conductance. These mirrors, however, degrade DC-output voltage operating range and consume chip area. In addition, above-threshold operation results in higher power dissipation.
Thus far, we have discussed the prior art pertaining to high power (typically greater than 1 micro watt) transconductors in above-threshold MOS and bipolar technologies. We now discuss the prior art pertaining to low power (typically less than 1 micro watt) subthreshold MOS transconductors. Two transconductance amplifiers well-known to those of ordinary skill in the art are described in "Analog VLSI and Neural Systems", C. Mead, Addison Wesley, 1989, pp. 67-82. Both have linear ranges of approximately 75 mV. Subthreshold MOS technology, like bipolar technology, is based on exponential nonlinearities. Thus, it is natural to employ source degeneration techniques to widen the linear range. Methods for getting wider linear range that exploit the Early effect in conjunction with a source degeneration method are described by X. Arreguit, "Compatible Lateral Bipolar Transistors in CMOS Technology: Model and Applications", Ecole Polytechnique Federale de Lausanne DSc Thesis, These no. 817, 1989. The Early voltage is, however, a parameter with high variance across transistors, and thus, one cannot expect to get good transconductance matching in this method. Further, such schemes are highly offset-prone, because any current mismatch manifests itself as a large voltage mismatch due to the exceptionally low transconductance.
The simple technique of using a diode as a source degeneration element extends the linear range of a differential pair to about .+-.150 mV, as described by L. Watts, D. A. Kerns, R. F. Lyon and C. A. Mead, "Improved Implementation of the silicon cochlea", I.E.E.E. Journal of Solid State Circuits, 27(5), May, 1992, pp. 692-700. However, it is difficult to increase this linear range further by using two stacked diodes in series as the degeneration element--the wider linear range that is achieved is obtained at the expense of a large loss in DC-input operating range. If one is constrained to operate within a 0-5 V supply, the signal levels remain constrained to take on small values because of the inadequate DC-input operating range.
It is tempting to think that the problem of getting wider linear range may be solved by interposing a capacitive divider between each input from the outside world and each input to the amplifier. (Of course, some form of slow adaptation is necessary to ensure that the DC-value of each floating input of the amplifier is constrained.) This approach is described by R. F. Lyon, "Analog Implementations of Auditory Models", DARPA Workshop on Speech and Natural Language, Morgan Kaufmann Publishers, San Mateo, Calif., 1991. However, the use of the capacitive divider implies that the signals entering the amplifier from the outside world get attenuated at its inputs, but the internal noise present at the amplifier's inputs remains the same. Assuming that the dominant noise is due to the internal noise of the amplifier, which is usually the case, the signal to noise ratio is worsened by exactly the same factor that the linear range is increased. Thus no gain in dynamic range whatsoever is achieved--the signal levels and the noise levels are scaled by the same factor. Further, the scheme did not work well in practice because of its sensitivity to circuit parasitics.